Both academic and industrial research institutes have extensively studied stable isotopic compositions of various elements in petroleum. The commonly studied elements include carbon, hydrogen, sulfur, nitrogen and oxygen. Isotopic compositions of individual components of natural gases, whole crude oils, crude oil fractions and solids have been determined. The most commonly studied element is carbon (13C/12C), distantly followed by hydrogen (D/H).
Mass Spectrometry
Various techniques for quantitatively or qualitatively determining isotopic composition are known. For example, certain types of isotope ratio determinations are commonly done on mass spectrometers that have been specifically designed for that purpose. Such instruments can determine the isotopic ratios of a limited number of gases: H2, N2, CO2 (for 13C/12C and 18O/16O ratios), SO2 and SF6. Therefore, when it is desired to determine the isotopic ratios of organic and inorganic compounds containing these elements, the compounds must be first quantitatively converted to the appropriate gas. Determination of HD/H2 and 13C/12C ratios in organic compounds requires the combustion of the sample (usually in the presence of hot CuO) to CO2 and water, cryogenic separation of the water from the CO2 and conversion of the water over hot metal shavings (U, Zn, Cr or Mn). In some cases, organic hydrogen is directly converted to elemental hydrogen by pyrolysis over hot metal shavings (Cr or Mn). The resulting gases then can be analyzed on the mass spectrometer. These steps typically require very labor intensive preparation procedures and the use of costly mass spectrometers specifically designed for isotope ratio determinations.
Specifically, for natural hydrocarbon gases, the gas mixture must first be separated into its individual compounds. This is typically accomplished by means of a gas chromatograph (GC) using helium as a carrier gas. The separated gases flow into a hot tube furnace (900° C.) packed with copper oxide, where the individual gases are quantitatively converted to CO2 and water. Once the separated compounds have emerged at the end of tube furnace, there are two methods to determine the isotopic composition of the individual compounds.
In the first, older method, the CO2 and water from the individual compounds are diverted into individual detachable cold traps, where they are frozen. Once all the components have been collected, the cold traps are transferred to a purification line, where the helium is pumped out and the CO2 is cryogenically separated from the water. The pure CO2 is then cryogenically trapped in a “transfer tube” and transferred to the mass spectrometer (MS) where the 13CO2/12CO2 ratio is determined. The water is transferred into a reduction tube, where it is quantitatively converted to hydrogen gas, which is then analyzed by the mass spectrometer for HD/H2 ratios. The whole process is labor intensive and only a few of the steps have been automated. The mass spectrometer is normally “tuned” to analyze CO2 samples. When enough water samples have been collected, it is “tuned” for HD/H2 analysis. Switching between CO2 and hydrogen tuning involves at least half a day of down time.
In the second method (called GCIRMS—GC Isotope Ratio Mass Spectrometry) the system is adapted to analyze either CO2 or hydrogen. For CO2, at the end of the tube furnace the helium/CO2/H2O mixture is passed through a water trap then, the helium/CO2 mixture is directly injected into the mass spectrometer where the 13CO2/12CO2 ratio is determined. This method requires significantly smaller samples, however the duration of the 13CO2/12CO2 ratio determination is much shorter (the width of the GC peak) resulting in less accurate ratios. The duration of the complete analysis (methane to pentane) is determined by length of time required to elute all the components through the GC. In order to determine the HD/H2 ratio, the water trap is removed and the copper oxide tube furnace is replaced by a pyrolysis tub furnace where the gases are converted to elemental carbon and hydrogen gas. The hydrogen gas is then analyzed with the appropriate tuning of the MS in a similar fashion to CO2.
The hydrogen isotope analysis is a relatively new technique and is not commonly used. All mass spectrometers have a relatively narrow dynamic range, which is often smaller than the range of concentrations that occurs in natural gas mixtures. Therefore, in order to determine all components using a GCIRMS instrument, it is often necessary to inject the same sample several times, varying the sample size with every injection to fit the concentration of each components into the dynamic range of the MS.
In addition, mass spectrometers and the associated preparation lines are costly, heavy and bulky. They include many glass and quartz parts and require continues supply of high purity helium and liquid nitrogen. Therefore, they cannot readily be “ruggedized” for on site field operations. Also, because of the length of time required for complete compound analyses, they cannot provide real time data, often needed in field operations.
Gas Chromatography
Gas chromatographs that detect the presence of hydrocarbon gases encountered during drilling have been deployed on drilling rigs for many years. However, these instruments are slow and often inaccurate and therefore do not permit instantaneous collection information regarding the hydrocarbon composition of the gases. No instrument that can be deployed to a drilling rig site for the purpose of rapid carbon and hydrogen isotope analyses is available on the market to date, let alone isotopic composition.
Isotopic Analysis Using Molecular Vibrations
Isotopic ratio measurements via quantum vibrational transitions have existed for many years. The two main measurement methods are based on emission spectra and absorption spectra. Initially, quantitative determination of isotopic ratios via the emission spectra of molecular transitions was inaccurate and not useful. At the same time, the absorption spectra lacked the resolution needed to characterize overlapping but isotopically different molecular transitions. This was mainly due to lack of power at monochromatic wavelengths. With the advent of lasers in the late 1960's, this limiting factor was eliminated, and high-resolution characterization of polyatomic species with isotopic substitution was possible. It was not until the completion of laser absorption studies in controlled laboratory settings that quantitative emission studies became useful. The primary focus of isotopic laser studies on short chain hydrocarbons, specifically methane, was characterization of spectroscopic properties for astronomical purposes. It was through these studies that the carbon isotopic composition of methane in extraterrestrial planetary atmospheres was first determined via emissions detected during near passes of artificial satellites in the late 1970's.
Laser isotopic studies of carbon in methane and other short chain hydrocarbons continued for academic purposes until the late 1980's. In the 1990's, little scientific work was done in the field of carbon isotopic measurements in hydrocarbon gases. Since the 1990's, the academic focus has shifted to laser isotopic studies of inorganic polyatomic molecules. To date, however, the only commercialized applications of 13C/12C measurements have been in medical research (measuring exhaled carbon dioxide), and in geological research for determining inorganic characterization of water and carbon in sandstone/mudstones, pyrite, sphalerite, galena and calcite. In the analysis of exhaled CO2, light emitted by a CO2 laser is used to measure isotope ratios. It should be noted that, to date, the determination of 13C for geological purposes has been applied only to carbonate rocks. Inorganically bound isotopes of sulfur, oxygen, and hydrogen have also been studied with lasers for geological and environmental purposes.
Analysis in Laser-Based Systems for Pollution Purposes
Determination of absolute concentration of trace levels (low ppb range) of atmospheric gasses using laser technology has been the focus of current academic and commercial research. Gasses currently studied are H2O, CO2, CO, NO, NO2, N2O, SO2, CH4, C2H2, HF, HCL, HBr, HI, HCN, H2S, O3, NH3, H2CO, PH2, O2, OCS, C2H6, C3H8, C4H10, although gasses above C2 have only been studied in laboratory settings. Of the hydrocarbon gases, only CH4 has been studied isotopically, and purely for academic purposes. Most research has been conducted for environmental ends, but detection of these molecules for monitoring leaks from industrial processes has also been of interest and commercial laser spectrometers for these purposes have been developed.
On-Site Field Analyses
Wells are often drilled for the purpose of extracting hydrocarbons from formations deep in the earth. In many cases, there is a variety of strata between the surface and the target formation. Some of these may contain hydrocarbons, of which it may be desirable to determine the isotopic composition. In addition, it is often desirable to determine the isotopic composition of the hydrocarbons in the target formation itself. Furthermore, it is often desirable to analyze isotopic composition during production, as well as during drilling and post-production. The downhole environment is very harsh, with high temperatures and pressures, and often corrosive liquids or gasses. The ruggedness of the downhole and well-site environments, their remoteness, and space constraints make it difficult to devise instruments that are capable of making direct analyses on the formation fluids.
Historically, the stability of the laser spectrometer has limited the usefulness of isotopic measurements via laser spectroscopy. New laser systems are available with very high repetition rates and better stability. Despite the existence of the foregoing technologies, a need still exists for very accurate real time hydrocarbon analysis in the context of drilling. It would also be desirable to provide a system for measuring the structural position of the isotopically substituted element.